What are Phase-Matched Cable Assemblies?

Phase Matched Cable Assemblies are a set of cables that have the same electrical length (phase length) or phase tolerance at a specified frequency (e.g. ±5˚ at 18 GHz). Many applications now days require multiple cables to be used together in a system. Take the example of an Electronically Steerable Antenna or ESA, where multiple sections of an antenna need to be connected to a system and be controlled. It is very important to ensure that phase-matched cables are used here to ensure proper performance, as each antenna element needs to be provided with a specific phase in comparison to the other elements. There are two ways to provide phase matching to cable assemblies. Cables can either be matched to each other based on a predetermined phase value, called absolute, or relative, where cables are bundled into sets with each set having the same electrical/phase length.

There are several parameters impact phase matching of cable assemblies. The main parameters have been highlighted below.

Parameters Affecting Phase Matching of Cable Assemblies

Frequency of Operation: Phase of cables change as a function of frequency. Low frequency cables are easy to match since the crests and troughs are widely spaced unlike high frequencies where the sinusoidal peaks become more closely spaced as frequency increases. The higher you go in frequency the more difficult it becomes to phase match cables. Hence, more labor time is required, thereby increasing the cost of manufacturing.

Type of Cables: Coaxial cables can be flexible, rigid or semi-rigid. Rigid are the most phase stable, followed by Semi-rigid and then flexible. When semi-rigid cables are bent to shape in certain applications, they have a natural tendency to maintain the shape compared to other flexible type cables. Thus, semi-rigid cables can provide a relatively better phase match than flexible cable assemblies. To maintain a better phase matching capability for cable assemblies in situations where rigidity is not allowed, a good amount of tweaking might be required.

Physical Length of Cable Assemblies: Longer cable assemblies allow a lot of bending at different angles and orientations than shorter cables. Therefore, shorter cables are relatively easier to phase-match than longer cables. Longer cable assemblies also undergo phase change during installation which may cause erroneous results during RF device testing. However, this can be later resolved by connecting phase adjustable connectors or adapters.

Connector Types: For a given cable assembly set, the same connector types should be used to avoid deviation of phase length between cables of the same set. Different connector types can be used, but then the cables would need to be adjusted to account for any phase difference caused by the different connector types. This is a complex, time-consuming and expensive process.

Surrounding Temperature: As the surrounding temperature changes, the dielectric properties of cables can change, thereby affecting the phase of the cable assemblies. Dielectric materials such as Poly Tetrafluoroethylene (PTFE) are mechanically robust by nature but have high insertion loss and hence, result in large phase changes. Cables that use air as dielectric are relatively less robust compared to PTFE, but exhibit lower insertion loss and hence, provide superior phase-temperature characteristics. In fact, at room temperature, PTFE undergoes a structural change that is known as “Teflon knee”, which affects the phase change when it moves through this temperature zone.

Changes in Propagation Velocity: It is clear that manufacturing without uncertainty is not possible and thus, the velocity is always specified with a tolerance such as V m/s ± 2%. This tolerance will cause changes in electrical length of cable assemblies. As a result, there will be phase changes. These changes are easily noticeable in longer cables or when cables from different sets are mixed.

Other Factors that impact the phase of a cable are the bend radius, the number of flexure cycles etc.

Phase matched cable assemblies are often used in integrated antenna receiver systems, military, aerospace, commercial radars, phased array radars, 5G antennas, test and measurement, telecommunications, industrial RF and microwave applications.